AMPS is an analog system that permits communication by mobile units operating within an analog cell site. CDMA systems, on the other hand, are digital systems that permit communication by portable units operating within a CDMA footprint; i.e., a geographical area offering digital CDMA service. In a dual mode system, the CDMA system is overlaid on an analog AMPS.
Since both systems operate contemporaneously in the dual mode system, some interference inherently occurs. Generally, there are four possible interference mechanisms between the AMPS and CDMA systems both operating in the 800-900 MHz frequency band (i.e., base station transmit frequency in the range of 869-894 MHz and receive frequency in the range of 824-849 MHz): (1) interference from AMPS sites to CDMA portable units; (2) interference from AMPS mobile units to CDMA sites; (3) interference from CDMA portable units to AMPS sites; and (4) interference from CDMA sites to AMPS mobile units.
FIG. 1 is a diagram of interference caused by an AMPS site to CDMA portable units operating within a CDMA footprint. As shown in FIG. 1, an interfering AMPS site located far from the CDMA footprint might still cause interference to CDMA portable units due to the AMPS site's excessive power transmissions, antenna height, or other operating conditions. In addition, if the interfering AMPS site is located close to a body of water, its transmission path across the water has a lower path loss, thereby permitting interference from an AMPS site located at a farther distance away from the CDMA footprint. Any AMPS site that interferes with the CDMA system requires clearing of analog channels from the frequency band being used by the CDMA system. Clearing is an expression commonly used to refer to the action of decommissioning that part of the spectrum from a cell site.
Even if the AMPS site does not cause excessive interference due to a very high path loss to any CDMA portable unit, for example, the path loss from a mobile unit within the AMPS site's serving area may still cause interference to the CDMA system if the path loss between the AMPS mobile unit and the CDMA base station is very low.
FIG. 2 is a diagram of interference caused by an AMPS mobile unit, operating within an AMPS site's serving area, to a CDMA system. In FIG. 2, the interfering AMPS mobile unit crosses a bridge, for example, within the AMPS site's serving area. Because the AMPS mobile unit is located over water, the transmission path across the water has a lower path loss, thereby facilitating interference with the CDMA system. Similar results occur when an AMPS mobile unit transmits from elevated highways, mountain roads, etc.
Conventional systems typically ignore such interference mechanisms. However, all of these interference mechanisms must be considered in a proper analysis of guard zones and guard bands. A guard zone is a geographical area in which cell sites must have a portion of their spectrum cleared to reduce interference between the analog AMPS and the digital CDMA systems. A guard band refers to the amount of spectrum, or frequency, that must be cleared at an AMPS site lying in the guard zone, when the digital and analog systems are overlaid. The size of the frequency band that must be cleared depends on issues such as transmit signal shape and mask and receiver filter structure and mask.
Conventional guard zone and guard band estimation methods are based on tessellated hexagonal grid cellular networks. These systems have found only limited direct application to real cellular network planning. In an analog system, cell sites are arranged in a hexagonal grouping and the "ring" of cell sites around the cell sites under consideration are referred to as tiers of cell sites. The typical result is that one or two tiers of analog cell sites must be cleared of the spectrum in order for the systems to operate over the same frequency band.
These conventional methods are idealistic and based on distance and a single theoretical path loss model. In addition, these methods do not consider factors such as user traffic, land use and land cover constraints, forward link margin (i.e., the value of a CDMA portable unit's forward link margin indicates the additional interference that the system can sustain at the specific location and still meet the link requirement), noise floor elevation (i.e., elevation in the effective noise at the base station due to the other portable units using the same frequency) due to multi-user traffic, or transmitter and receiver filter characteristics typically referred to as masks. Because the conventional methods do not consider these factors, they do not result in very accurate results suitable for real-life CDMA cellular deployments.
Therefore, a need exists in networks to efficiently determine the number of AMPS sites which need clearing of the CDMA frequency band while minimizing interference between the AMPS sites and the overlaid CDMA system.